Might suggest squats to at least keep muscle mass and a higher level of strength. Strong legs are still important as we all get older. Doesn't have to be a lot of extra work for the legs, 3 X 5-7 reps, a couple times a week should do it. Really don't need higher reps if your doing the treadmill thing that many times a week. Squats also tend to improve the balance, rather than doing a leg exercise sitting or lying.
Can replace squats with step-ups, holding a DB in hand or a BB across the shoulders. Suggest also an exercise like GoodMornings, extensions, Romanian DL for the important lower back & ham string muscles. Might find the stretch also may improve the running abilities. Keeping them healthy in the process.
With running (treadmill or track), as excellent as it is, the leg muscles are only getting a very, very short ROM. Unless you switch to cross country/trail/hill running, etc. If you don't want to include any direct ROM movements for the legs (squats, leg presses, etc) than might throw in some sprints, like 40 yards or longer.
Some of the better calves are on runners (men & women) who run hills and mountain trails. I ran mountain fire trails when getting in football shape off season....great for stamina (endurance plus strength).
Just a personal view; always preferred running out doors than on a inside track or on a treadmill. Good Luck.