I understand that exercise after diving can be a bad thing wrt nitrogen.
Are there any guidelines for how much exercise? Is a half hour run a couple of hours after a shallow dive significantly increasing risk? Should I ask somebody else to carry my gear out of the water?
Guidelines with respect to exercise after diving are not based on particularly strong evidence. I consider Ern Campbell's recommendations to avoid heavy exercise 4 hours prior and 6 hours after (with respect to no stop dives) a logical 'best guess' position. He bases these numbers primarily on a 1997 study from NASA looking at bubble development after exercise in astronauts (1).
A paper looking specifically at exercise in divers after diving was authored in 1949 and resulted in a blanket ban on exercise during or after decompression in the US Navy until the 1970s (2). (They found a 34% increase in DCS incidence in divers who lifted 12kg weights for 2 hours after
no stop dives at 12, 30 and 46m - note the discrepancy in the title).
(1) Dervay J,Powel MR, Fife CE. Effective lifetimes of tissue micronuclei generated by musculoskeletal stress. Aviat. Space and Environ. Med., 68 (Suppl), A12. (1997)
(2) van der Aue OE, Kellar RJ, Brinton ES. The effect of exercise during decompression from increased barometric pressures on the incidence of decompression sickness in man. Report 8-49. US Navy Experimental Diving Unit, 1949