Let your aerobic speed (speed at MAF) be your guide. If you start lifting weights and your MAF tests start to decline, or your resting heart rate gets elevated for a prolonged period of time, then bad. If you keep on improving and RHR stays the same or slightly improves, then the weights aren't a problem. Mark Allen did strength training in his later years during the base period and had no problem. Twice a week with 2-3 recovery days in between (from his book Fit Soul Fit Body). Just follow your MAF tests and RHR and you can't go wrong.