Two things get tired when you run -- your lungs and your legs. Working both while focusing on more muscle stamina will help with those longer timed runs or rucks in your future.
Here is a good running / leg stamina / muscle endurance workout for you to mix into your leg day routine when in a running cycle:
Warmup 5 minutes / stretch legs (bike / jog / elliptical – your choice) Run 20 minutes - fast - how far do you get? THEN: Do the Squat / Run Interval Pyramid Run - Squat Pyramid Run 100m fast - 1 squat - run 100m 2 squats - run 100m 3 squats – run 100m Keep going up to 10 and then continue back down the pyramid to 1. This is a total of 100 squats mixed with 1900m of running at a faster than jogging pace. Swim – warmup 500m with no fins – This is a good way to make any future 500m or 500yd swim test "just another warmup" Then Swim for 30 minutes with fins non-stop for max distance. How far do you get? OR ruck for 30 minutes - how far do you get?