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.