Can I take it you managed a 6 mile tempo effort at around 7:30/m then? If this is the case, it would suggest that your marathon time is soft compared to what you're capable of at shorter distances, e.g. from 3:51 your 10k equivalent pace is 7:55 but you can clearly run quicker than this if you're virtually covering the distance quicker in training with something left in the tank. If you haven't got any recent (good) race times over 5k - half marathon range I would suggest scheduling a fast 5k or 10k to revise your training paces, and in the mean time just use perceived effort for your tempo runs. Anything around 3 - 6 miles continuous at a "hard but not killing yourself" effort level will be working somewhere around your threshold so you'll definitely benefit.