So what your looking at is 3 hard days, 3 easy days, 1 rest day
Hard days are 20-25km, Should take 50-60mins. 8-10km warmup at a slowly increasing pace to hard, then either of the following
- 3 x 250metre sprints all out. Rest between sprints are 3km rolling recovery. Roll down is 5km
- Find a 5-8% gradient hill 1.5-2.5km long hill approx 7-9k from your house. Do 2 hillclimbs at vo2max level. i.e. your only just starting to blow up completely in the final few hundred metres (92% max of HR), recovery is roll down hill, roll off hill for a few hundred meters, turn around and repeat, going hard by start of climb. Rolldown is return to house. (the 2nd time up will hurt bigtime!)
If your sprinting, use the same circuit, with the same sprint points. Note your max speed, and also note any winds you encounter during each sprint.
If your hill climbing, use the same hill, time each from a start point to an end point.
You will see improvements over time if you keep a diary.
As far as distance is concerned. If you can do a 2 hour ride at tempo pacing 50% of the time, this will also result in good adaption. (85% of max HR for half your ride distance). i.e. warmup for 10-15k, then ride at a level that has you needing to open your mouth to breath. Do an initial 3-5k tester building up to 85% HR. Rest for 1km. Then do 10km effort at tempo pacing. then rest for a few k. Then do 20ish k at tempo pacing. And then recover, and add additional efforts depending on distance. Roll down is 5-10k.
All these hard days need a easy day either side. 20km on flatish ground at recovery pace is ideal.
If your really strapped for time and can't get a distance day in, replace it with a lactate threshold short distance day. i.e. 25k, 5-10k warmup, then tt for 10-15km at ftp (89% max HR). 5k warmdown.
/\ p.s. I use to do this kind of training when I was racing u/17 and had school and thus minimal time to train during the week /\