I agree that ambulance is the hardest vehicle mission but it gives you infinite sprint.
I tried that mission over and over failed a few times on level 10 and 11 and two times on level 12 , but when i finally passed it, I was sprinting all day , didn't bother to borrow a vehicle. I suggest you to use the ambulance from the hospital near north point mall as this is the easiest (I tried every hospital in VC a few times) and I aslo completed that mission from it.
I started to play San Andreas a few days ago and now I'm trying to do the R3 missions first, as this is a great benefit in later missions.
I have only one question for those who completed R3 missions from SA.
What bonuses do you get after completeing certain levels of R3 missions?