For me the key has been doing spin bike / turbo trainer sessions in between “real riding”, in training for 250 mile road ride in 24 hours and with limited time due to having a young family, i had a 5 day a week ride schedule of one long ride at the weekend (road or MTB), my 60 mile round trip to work once a week, so two long rides done a week, interspersed with 3 sessions on the turbo a week for an hour after putting my daughter to bed of:
1 – 10 minutes warm up, then constant 40 minutes of HR at 90-95% and cadence of 95, then 10 minutes warm down.
2 – 10 minutes warm up, then 30 seconds as hard and fast as possible, 30 seconds recovery and repeart for 40 minutes, then 10 minutes warm down.
3 – 10 minutes warm up, then my “hill climb” sets of 3 minutes in a hard gear i can just about maintain a cadence of 85rpm in, then 2 minutes recovery, for 40 minutes, 10 minute warm down.
I know that the perceived knowledge is to put in the “base miles” for building up staminor, but I found that doing the turbo trainer sessions was what really brought my fitness on leaps and bounds.