Here are some tips for flying XC, I've spent a lot of time flying from the back of pick-up trucks!
You want to be at 3,000 or higher before you start, especially if you are going upwind first.
If you have a choice, go downwind on the out leg. Get in a thermal, get high, stupid high, and THEN start down the course riding that bubble. When it poops out, go down the road smartly. Going downwind will help adding windspeed to ground speed and you will catch up to thermals. If your altitude is still way high, slow down in lift but keep going, do not circle. You want to get to the goal with the vehicle prior to the sailplane's arrival so you don't end up overflying the goal (big rookie mistake).
Going upwind on the return, keep the model ahead of the vehicle at all times. When you encounter lift and need to work it, stop the vehicle immediately, and only thermal back to the truck, do not give up ground unless the lift is super and you are low. If you have good altitude, slow in lift but keep going.
Your goal should be to get high and stay high. Surf the lift but don't stop and never give up ground already covered (it's the same as looking for air in the same schmegg twice).
Oh, timing of the attempt, it's best to use the "Noon Balloon" time frame, big air going out, and usually less wind on the way back as the afternoon progresses. If there isn't much wind, so much the better and this doesn't apply as much.
The above applies if you venture upwind first (depends on the course(s) available to you), it will just seem so much longer seesawing your way upwind.