"...sitting on Mars for nearly 2 years while sitting in a lava-tube doesn't seem like a hot idea."

Suppose that on average the crew on were in a lava tube for 20 hours of a 24 hr day. So leaving up to 4 hours per day outside the lava tube every day [totaling over 2800 hrs in the field over this 2 year period]. This allowing that on some days you wouldn't leave your hole, and other days you could be outside for 6 to 8 hours.
Considering all the prep time needed to go into a near vacuum, and all the documentation one would need follow on, plus you need to sleep, eat, do the laundry, repair maintain equipment, etc. It seems to me one would more than enough time spent on the surface of Mars.