I know a different gospel than others here. Went to different schools. Pain in back of knees - saddle too far back. Pain in front of knees - saddle too far forward.
Pulling a hamstring sprinting is unusual. More common would be throwing out your lower back. In fact sprinting causes lots of problems of which a hamstring pull barely makes the list. OP sounds like a very active person used to playing hard. >Cycling Is A Technical Sport.< Yes, you can just hop on a bike and go for a ride. Going for a hard hamstring-pulling sprint with uncertain equipment and not much warmup, yeah, that could cause problems. Three laps of Central Park is barely a warmup even if you structured it just so.
Knees heal real slow. Pulled ham is not a quick one either. Give yourself a break. Instead of asking online find someone to ride with. Maybe someone older with race experience. You are having too many problems and sounds like time to start over from scratch.