Base rebuilding

I have, reluctantly, accepted that there will likely be few opportunities to ride outside until March. I do not have a trainer, so this limits my training opportunities to spinning class. For THIS year, that is likely not a bad thing as it is providing the chance to make certain my leg is healed and to work on the hip problem.

Considering I've been down since mid-September - with only the occasional 15-20 mile ride in a rather hilly park, I've lost my base. Most of it at least. I am assuming that it won't take as long to build it as last year - but is this an incorrect assumption?

I have started to hit the spinning bike about 30 minutes before class starts to work in some extra time building endurance - am working with my trainer on this one. Right now I am focusing on getting my heart rate to a certain level and keeping it there for a certain time period and staying in the saddle, no climbing or standing runs or anything else - THOSE things are for class right now. I need to start with rebuilding my cardiovascular endurance.

Given my propensity to over-do things and to push myself too hard, I need to develop a realistic approach to my training once I am riding regularly, and am working with my trainer on what I can do before then to support my base building. My injuries are healed, but I also am quite aware that until I rebuild my strength that I am probably prone to re-injury if I am not careful.

Any tips or advice? My first real goal of the season is to hopefully ride the Horsey Hundred - though we will see if I will be ready for a century by Memorial Day. Last year I had my base up to 75 miles, but it will depend on my starting point this year. I am not proud, nothing wrong with a metric century :)