Imagine the Apollo follow-on wasn't Skylab, it was the construction of a Mars ship.
Back in the early 70s they had absolutely no appreciation for the radiation environment. Aerobraking on Mars was considered unworkable by some and trivial by others - Viking would prove both wrong but not until the mid 70s. Nuclear thermal was already a political dead duck but maybe it could have been resuscitated for an EOR mission configuration, but it's still impulse, ion engines were shelved in the early 60s as impractical, hall effect thrusters were secret Russian business. Long duration spaceflight was a backwater of scientific knowledge, as was most space medicine - some would say it still is - the bone loss problem was certainly an unknown unknown. In-situ resource utilization was a complete non-starter.
So, the good news is they wouldn't try to build Battlestar Galactica. They'd go with a capsule and a lander, but bigger and heavier than the Apollo LM and with a heat shield. They'd use a nuclear rocket which means they'd need to develop near-zero-boil-off cryogenic stages (woo! we don't even have that technology!) both for return and to mass hydrogen propellant in Earth orbit. The mission would be opposition-class and the goal would be flags and footprints.
As was always the case back then, the engineering would be in advance of the mission plan. Those nuclear stages would be ready to go before the need for them. The shakedown cruises would probably be to lunar orbit and they'd be disposed of on the Moon with a chemical stage providing the final insertion into Earth orbit. The Viking mission schedule would have been advanced to act as precursors, scouting for landing sites, etc, much like the Surveyor missions did for the Moon. The lessons learnt on the heat shields would be integrated into the lander modules.
By the late 70s the interplanetary cruise duration would be known and in the early 80s the rude shock of long duration flight effects on the human body would be so obvious that even the damn-the-doctors-full-speed-ahead culture of NASA in those days wouldn't be able to deny it. Bone loss would be a significant threat to the continuation of the program. If astronauts have lost 80% of their bone mass by the time they get to Mars, they can't land and they can't plant the flag.
A short arm rotating inflatable habitat would be the most popular solution. Ever since the 40s it was considered the obvious solution to providing artificial gravity and the only thing keeping it out of the program so far would have been the importance of establishing the Mars program as separate to the Station program, which would be starved for funds by now. Begrudgingly, the two programs would be merged, but the delays would add at least 5 years to the schedule.
So, around the early 1990s the epic voyage to Mars could begin. It would take 1.5 years to travel to Mars. After a 3 day checkout in Mars orbit the crew would descend to the surface where they would plant the flag. They would spend 12 days on the surface with regular resupply from the crew left in orbit, giving them time to collect samples, and do some exploration with Apollo-style open rovers. Leaving that single site they would return to orbit, to aid the 5 day preparation effort to return to Earth, another 1.5 years later.
By the mid to late 90s the program would be canceled after a single flight. The crew habitat and spent propellant tanks would be re-purposed as a low Earth orbit space station. Perhaps there would be some cooperation with the on-going Russian space station program. By early 2010 perhaps people would be asking why we never went back to Mars.
This is my alternate history, if you'd like to read another account, you can't go past Stephen Baxter's "Voyage" which speculates on a NASA where John F. Kennedy was merely wheelchair bound not killed. It's a great read, check it out.