Martin318is said:
Fitness does improve over time IF you keep adjusting your training to take advantage of those improvements. Rest is as important as time on the bike and taking time off to allow your body to adapt will assist your progress. Consider doing 3 weeks where each week is a little more kms than the week before and then in week 4 drop back to about 75% of that. Then start the next 4 week block from about where you were in the 2nd or 3rd week. What you should see is an up and down wave pattern of ride durations that climbs over time. Ideally you would want to end up doing some 4+ hour rides per month and be riding over 3hrs regularly. If you do the same pattern every single week then you will only really be training yourself to ride that distance per day. variation of effort and duration are both very important.
The schedule being described is excellent advice. My version of it is a little different.
1: Figure out how many hours you actually will commit between now and the ride and still have fun the whole way along. Use that maximum hours per week as your largest block of time. The other three periods are shorter, but more intense. Volume goes up, intensity goes down. Don't try to increase both at once.
2. Once every 4 weeks do a 4+ hour ride. You will practice keeping yourself fed and watered on this ride so that when you do the big ride, you do not have food/water related issues. Try to set a tempo and a high cadence and keep it all the way through. The last hour should feel pretty bad.
3. There is no substitute for intense training efforts. The beauty of them is about 10-20 minutes is all that's needed on a ride. No cheating. Max efforts in bigger gears on a hill lasting about 2 minutes. Do the hill three times and you are done and you should FEEL done. Keep it fresh. change the hills, change gears, but keep the cadence low so you feel it in your muscles. Spin home. Intervals at various cadences are nice too. Less really is more here.
This is where Fergie and others have an apoplectic fit.
If weather does not permit a ride, the gym is great. 10 minute warm up at progressive intensity on one of the fancy exercise bikes with varying resistance. Followed by circuit training for the whole body including squats with free weights. Keep the heart rate up while working weights. 15-20 reps to failure.....
Every body is different. So, fashion it to work best for you. I've had great success in a very tight schedule using these methods. The rare long rides go just fine.