Start with commuting.
I know you can't work up a real sweat in the school uniform, but any kms done on the wheels will help condition the leg muscles.
You don't need to do 50km rides to get fit and fast.
Try and set aside a block of time rather than a block of distance and focus on your weaknesses first.
You will have to be brutally honest with yourself and work out where you need to do the most work. If it's a consistent endurance you are after then you really need to get some time in the saddle. But not hour after hour.
Also, you need to be realistic about your current fitness and not bite off too much to begin with and injure yourself. I've been there, and it sucks.
Start with making a training schedule you know that you can stick to.
There's no point saying "I'll do three days a week when I can get a day when theres nothing on' because there will always be 'something' to distract you. You have to make training your 'something'.
Break the week up into three days doing different intervals.
Say:
Monday. 4pm - 1 hour ride on tarmac spin session.
First half hour get your heart rate high, and every five minutes do a 15 - 20 second sprint at full pace. Turn around and for the next half an hour keep your heart rate medium/high and just lock into a rhythm and point yourself home.
Thursday. 4pm - 1 hour ride on tarmac medium spin.
Just concentrate on getting locked into a rhythm for the entire ride. Try not to stop unless it's for traffic lights or if you feel a cramp or rest is necessary.
Get into a gear where you can sit on a cadence of about 110 beats per minute.
Just try and make sure you do everything you can to keep the pedals spinning in time, like a clock. If you need to go up or down a gear to maintain this rhythm then do it. Just try and keep that pace on the pedals. Don't worry if your speed or average speed goes up or down, it's the pedals you want to be in time.
This will condition your legs and muscles very quickly to build up a tolerance to this exercise and increase endurance. It will not increase strength and speed (that's what the Monday ride is for).
Sat - 7am / 8am. 1.5 hours mixed spin session on tarmac.
Spend the first hour doing Thursdays workout, then for the last half hour, turn your bike around and do Monday's workout. There should be about ten to twenty minutes at the end for a wind down and just spin your way back home for the last few minutes at your own pace. Always warm down after a session.
This is what I would do for the first Month, just to get my body used to working in this way and so it can acclimatise to it's new duties.
For the next month, I would be working on maintaining what I have built in the prior month, and adding some exercises that were more specific to riding in a recumbent style. I have never ridden a recumbent so I would have to research what muscle groups are used. I imagine core and lower back would be providing some power, but I don't know, just guessing there.
Of course, this is what I would do if I wanted strength and endurance, and is only an example of a formula to obtain this, if you want pure speed then you will have to work a lot harder.
Three days a week, three and a half hours.
Eat well, sleep well, rehydrate, and don't get distracted.
Give it a crack.
