The British Cycling 20 minutes warm-up is ideal for a high intensity race where you will go “full gas” from the start of a road race, a cyclo-cross race or straight into a high intensity training session.
Just going out for a ride, you should start your ride at an easy pace and increase inrensity during your first ten minutes of the ride.For a time trial you may want to warm up to the heart rate or power to match your time trial pace.
Do not hop on your bike and go full gas. This will put a lot of strain on your heart. It is far wiser to allow your heart rate to gradually increase to your target pace. Many experenced riders will bring a turbo or rollers as it can be difficult to warm up at the venue. Hill Climb Strength Exercises for youth and Adults
There are three videos I have found on the British Cycling website that you might like to watch.
CAUTION: If you have an injury check with your doctor and or a Sports Injury Physiotherapists first
First I would like you to read the following extracts from the Mayo Clinic article on Strength training is OK for Kids? Try also to make time to read the preceding full article link
I am not a qualified gym fitness instructor, but I have worked with youth for a number of years. Development of technique is very important. Weights can be replaced in elastic exercise ribbon or bands.
Be also aware that: In growing youth most power will come from natural physical growth, so setting power goals may be unhelpful for a younger rider. "Intensity should be based on feel. We don't use power at junior academy, efforts are just efforts — you go hard [by feel] or you don't go hard." Tom Pidcock, Cycling Weekly, Dec. 2018.The following British Cycling videos are not necessarily made with youth in mind. I strongly recommend that youth do exercises that utilise their own weight.
This article is mainly aimed at youth and juniors, (under 18)
The general census is at least 3 sessions a week, training 4 or 5 times a week is ideal.
www.cyclingweekly.com suggested: Regular short rides are the fastest way to boost your fitness.. 60+minutes long ride. Hilly Ride 30-60min, Hard ride30-60min. (around 4 hours total)
www.cyclingtips.com Ten hours of training per week is a good amount for most serious recreational riders: Monday rest; Tues. 60-90 minutes 90%HR. Wed. 2 hours endurance. Thurs: 60-90 minutes with a few mates push each other. Fri. One hour coffee ride with friends, Finishing with a short upper body weight training session, swim, Pilates or yoga. Sat: 2 hours include hills. Sunday, two hours endurance.(10 - 20 hours adults)
Dr Laura Dugas, Young athletes whose training hours exceeded their age or whose sports hours exceeded their free play by a factor greater than 2 were also more likely to develop injuries and overuse injuries.
HOW AND WHERE TO TRIN
Here are key points from my research below :
Next: read this article first from Cycling Weekly. Talent or training? How a gifted young rider becomes a champion
Then do a Lifestyle Audit (word) to see how many hours you can train a week.
If you want more information read below there are lots of informative links and information.
PDF of full report: Risk of Injuries Associated With Sport Specialization and Intense Training Patterns in Young Athletes: A Longitudinal Clinical Case-Control Study
Quotes from Michael Guilford a British Cycling accredited coach who specialises in developing young riders. interviewed by Cycling Weekly.
“You need to be careful with expectations of physical improvement in young riders. Development in power is likely to come mostly from natural physical growth, so setting goals like ‘improve 10-minute power from 200W to 300W’ may be unhelpful for a younger rider. It might be more appropriate to focus on a different area such as cadence.”
“Building good habits is more important than hitting numbers, since the numbers will change anyway as the rider grows… the best habit for a top-level cyclist is training to be a better bike rider”Do you use Zwift during a live coach led training session and have problems with terrain changing your resistance when you are trying to hold a set resistance?
Using a RIDERS CHOICE workout on Zwift will flatten out the road. Your smart turbo will then maintain the same resistance on terrain, allowing you to chose your own gear and cadence to set power. You will still see your power setting etc.on the screen.
This is how to use Riders Choice during a Zwift Group session:
Off the Bike Climbing Strength Exercise for Cyclists:If you struggle on the hills, this cycling climbing strength routine from EIS strength and conditioning coaches Scott Pearson and Joe Hewitt could help. They demonstrate correct form for mountain climbers, renegade rows, straight leg deadlifts and Bulgarian squats. Follow their advice on how often to train and what numbers of sets and reps to perform and you will soon be flying up those climbs.
Off the Bike Back and Lower Body Mobilisation Routine for Cyclists: Including mobilisation work in your training routine will make you more resilient to injuries, make you more comfortable on the bike and improve your performance. Cycling many health and fitness benefits but, on its own, has very limited movement patterns. This upper body mobilisation routine will help reduce tightness and make you less prone to injury.
Off the Bike Upper Body Mobilisation Routine for Cyclists: Very simular to the previous routine except concentrating on the upper back,help reduce tightness and make you less prone to injury. 
Strength exercises for cyclists. For improved performance, health benefits and injury prevention, all cyclists should include strength work in their training routine. EIS strength and conditioning coaches Scott Pearson and Joe Hewitt demonstrate the key movements suitable for people new to strength work including goblet squats, lateral squats, stiff leg deadlifts and front planks. They also tell you how often to train and what numbers of sets and reps to perform.