>My gut is that the Enterprise is the first one to survive a
>5-year mission because earlier in their career the ships
>weren't sent out on ones that long. Probably because when they first
>arrived on scene they were needed to deal with the difficulties of the
>early 24th century, and then when they were sent on missions they
>weren't sent on ones that long either because they didn't know if they
>could manage that long or they didn't want to risk having ships of
>that caliber too far away to get back quickly if needed.
>
>By the time things had settled down to a dull roar and Starfleet was
>confident it could plan on having Connies out of reach for that long,
>it was well into the latter half of the 24th century. And even then,
>5 years proved to be a long time for one of them to be away from
>support, etc. Who knows how many of the 5-year mission ships were
>original Block I, or were later Blocks, but Enterprise was
>clearly the first of any block to return (mostly) intact...

It may have been different in the UF-verse, of course, but in ST:TOS, it wasn't 5 years of constant exploration, nor was it without support from Starfleet and civilian bases. Enterprise made a number of stops at starbases: it was headed to Starbase 6 for R&R right before encountering the space amoeba that destroyed Intrepid, was carrying Commodore Stocker to Starbase 10, and stopped at Starbase 11 and Deep Space Station K-7. Enterprise was diverted from exploration duties a number of times to carry out short-term military and diplomatic missions, served as a testbed in The Ultimated Computer, ferried scientists and/or bureaucrats to various places, and appeared to be making brief visits to Federation outposts and colonies as it moved between the bases and the zones it was exploring.