Hi Pete. Do one weight training session per week and don't use really heavy weights, you want to be a strong runner not Arnie Schwarzenegger. Do a basic set of exercises - start with 1 set of up to 10 reps building to 2 sets of 12-15 reps. Major muscle groups (chest, back, upper legs) - bench press, squats or leg press, lat pull-downs, leg extensions, leg curls. Medium muscle groups (pectorals, deltoids, gastronemius) - dumbbell pullover, shoulder press, calf raises, upright rows.
Small muscle groups (biceps, triceps) - Biceps curls, triceps extensions, lateral raises. And don't forget your abdominal exercises! Courtesy of the triathlete with the most ever Ironman wins, Paula Newby-Fraser, it's part of Paula's '7 Months to Ironman' training plan and it really works.
Have fun!
Sheila Anne.