If your tendency is to gain fat easily (i.e. slow metabolism), add in one additional day per week of duration work, entailing about 45 minutes of a steady, challenging pace. Your heart rate is of no concern, nor is your method of madness (e.g. run, stationary bike, elliptical). If your tendency is to lose muscle (i.e. fast metabolism), add in one additional day per week of interval work, entailing 20 minutes total of 30 sec / 90 sec sprint/recovery work.
Those statements are maybe a little unqualified just by themselves.
I mean that in the sense … what is the aim? Sprinting?, football?, soccer? or just body comp? What is the starting BF%?
Personally while I don’t like it at all - I believe that if it must be done - any steady state aerobic conditioning or running should max out at 20-35 mins especailly for sports such as rugby or soccer.
I would regard it as starting to become catabolic and negatting speedwork after that if those are important to you.
Of course the BF% must be relatively low to start with - for those over 25% any steady work for a period of time will be quite hard for them.
A few changes to diet and even a slight increase in activty will make changes for them.
For fat loss for those reasonably fit - I would stick to something like 400m interval runs CF Tempo work, or my personal for sports people - Tempo-like Intervals of 150-200m with a 50 m walk, 100m jog and 25 m walk around to the start again.
If you’re a sprinter the Tempo-work knocks fat off anyway so I don’t see it really as a concern for sprinters.
Of course underline all fat-loss comments by stressing the importance of diet and especially protein intake for LBM.
That is relevant only for bodybuilding but it should be the opposite. One with the slow metabolism should be doing the interval work because it increases 24 hour energy expenditure a lot more. One who tends to lose muscle easily should stick to low intensity steady state work…interval work increases the risk of overtraining and burns too many carbohydrtates.