Mary-Ann can be found in Vinewood Hills, on top of a large hill with a radio tower ("F" with a little question mark on your map if you've done any other missions with her). After some choice dialog with her, you'll have to race against her on a bike. Although the race isn't too challenging in itself, Mary-Ann is still a tough opponent and you'll likely need close to 4 bars of stamina to compete without getting tired.
There's one or two tricky spots on the course, mostly due to narrow pathways, but most of the course is pretty easy. When you get to road areas, make sure you watch out for vehicles, so you don't get knocked off your bike. Once you finish the race, the mission will be over.
