Let me guess... you are getting blown up before you even make it to the part on foot? Too much car damage and you die before the last jump?

My advice... Make sure you kill the last two guys...they seem to be key to making it. I finally did once I was able to shoot both of them. The two I'm talking about are once you come out of the water spill-way, there are three Cholos on the left... then you will hear him say "Here is one last Lance Vance Chance..." (or something close to that) then there are two more Cholos. This is where I would always fail the mission (from car damage) until I killed those last two...