Maddog mission ........??

You must have used a cheat before that mission and saved the game. Thats what is causing the problem now. You have no other choice but to start the game all over again. But if you know where you used cheat u can go before that and try again.

Its not that difficult, i've done it and never failed like you are saying. You have plenty of time to get the truck and go right below him. What you should do is Use the Sprint button to go to the pick up truck as soon as the cutscene ends and then drive slowly without crashing and you should be able to make it in time. If you still have problems let me know the exact problem why you are not able to make it in time.