When I started training for muscle size , I was on the back of a half marathon , so I was still running a couple of times per week . I found the accepted wisdom was correct it ' s tough to build size whilst doing cardio more than about twice a week but really , the main limitation is how much you can eat . Certainly , I found it really tough to build much muscle at all while still running one 10 K and then generally a 1012 K per week , just because I couldn ' t consume enough lean calories . I didn ' t have a lot of fat to burn when I started training for size because I ' d been training for distance running , so that obviously plays into it for you as well if you ' re carrying a little bit of extra weight you ' ve mentioned you want to burn fat , then the calories are initially less of a limitation . But the biggest thing I ' ve noticed is that it ' s HARD to run with the extra muscle mass , even when against the same weight in fat . I ' m carrying maybe 6 % more muscle mass since I stopped running very much , and now when I do run , I find running absolutely BRUTAL because of course in addition to carrying the actual weight , my body now needs to pump blood and oxygen to more muscle . The point being that in some ways you ' re making it very hard for yourself by doing both I definitely struggle . That said , if you just want to build strength , and not size , then you ' re probably fine with the amount of cardio you ' re doing . Some gratuitous advice , though You ' ve said you want to do a 5 K and then a 10 K run . From the perspective of reaching that goal , I recommend that you run longer , but slower , and probably a bit less frequently three times a week , for half an hour to an hour , say . Most people starting out running including me , when I started will go too fast and burn out too early in the run . 15 minutes won ' t really cut it for the running goals you ' ve set you need to run for a half hour , no matter how slow , because that ' s the minimum amount of time the 5 K will take eventually . The other way to do it is to run the distance , no matter how long , but that can be daunting , I know . Conventional wisdom says you should also do speed work once or twice a week , but the point is that , to do distance , you need to initially ignore speed . Either way , good luck !