Are you 16 years old? Hamstring pull bad enough for a doctor visit in July and you are doing centuries plural and intervals by September? If you push yourself all the time with no restraint yes you can have problems. Did we ever get as far as determining why the initial hamstring pull occurred? Pulling a ham on a bike is not an easy thing to do, if it occurs there is a reason. Or maybe the ham pull was not on the bike? Can't tell.