I agree with Damien. Those long runs are less likely to produce hamstring strain, but the enhanced need for circulation makes them more vulnerable to what happens in a car seat, or in any seat, for that matter. Car seats in the US tend to make things hardest on the right hamstrings, because of the standardization of gas and break pedal use on the right side. Although I don't recommend it for safety reasons, I've been training myself to use the left foot for both to reduce the hamstring pain. Works great, except I almost had an accident when my reaction was to slam on the breaks with my right foot, lol.

I also agree that stretching is overrated, especially for marathoners. If your hamstrings are tightening up from overuse, forcing them to elongate is not the answer. Ensuring adequate circulation throughout the day, is.

Bottom line: be aware of what your hamstrings are up against during all 24 hours of the day, not just while training. Mileage could be the least of your worries. One aspect of speed training to look out for is track workouts, which are harder on the right hamstrings when run in the traditional counter-clockwise direction.

In the meantime, do what you can to enhance circulation in your hamstrings, which may be starving for blood flow after an average day, even when you are not training. Remain seated as little as possible, and move around frequently throughout the day. When driving for extended periods, take advantage of every break to elevate and massage the underside of your thighs to bring circulation back into the hamstrings.