As well as the advice you have gotten here (good advice all,) look in to the Master's Racing Forum, Masters Racing (All Disciplines) , even if you aren't wanting to race. Those folks are knowledgeable and ready to help when asked. I have gotten good recovery advice from them when I came back in to riding.

Best of luck with gaining your speed, please listen to those that recommend letting the URI, or whatever your illness is, get completely over and gone.

Bill