I too recall that Gru was believed to be in excellent health, died very suddenly and gave no indications of any ill health leading upto his death.
I recall stories that appeared in the Bullpen after his death confirming that he ran marathons for charity and would do ridiculous things at marvel get togethers like handstands and backflips, all of which would indicate he was in great shape.
Whilst I dont think his health could be used as an excuse for his later work on Cap I also think many fans are overly critical of that work. Had it occurred in the middle of his run rather than at the end I dont believe people would have cared or remembered. As it is 'Fighting Chance' simply became know as a poor story that occurred immediately before Waids excellent first run on the book. In my opinion it wasnt that bad overall.
You really have to view Gruenwalds run as a whole to appreciate why he was such a great Cap scribe and its a shame that things like Fighting Chance have been recently reprinted where as the Bloodstone Hunt and Streets of Poison have been out of print for years and to my knowledge the Replacement Cap arc has never been reprinted.
Just the whole idea of Cap dieing but still fighting the good fight (okay in armor, ugh). When life's is really beating me down I admit I think back to the spirit of that arc and think "All I need is a fighting chance"