Somthing similar happened to me. Was very fit then fell off the bike for a few years. A year ago tried to get fit fast using my HR, doing intervals etc and ended up with Aterial Fibralation for a week,visiting cardiologists and all sorts at 31. Dont push yourself too hard, go very slow. LSD, lots of long slow distance. I slowed down and got fit slowly over 6 months and have lost 6KG's Keep your heart rate down below 150 BPM , 135bpm sounds good. Dont rev the engine too hard too fast, build up slowly over a 12 to 16 week period before you start scrutinising you performance too much. Good luck and enjoy pushing those pedals.
