eh. if you want to excel at endurance training the only way is to run more.
unfortunately testosterone isn't gonna help you, it will hinder. take it from a former competitive runner who tried to run on a cycle. higher bp, higher heart rate will cripple your times. also the pumps you will get in your shins and calves will damage you. i ran a 5K on a cycle before. had to quit halfway through. the pumps were ridiculous. people were freaking out cause my calves looked like they ballooned into a blimp and my veins were popping out.. they thought i had a monster cramp.
try it yourself and see. maybe there is a perfect dose where you can avoid the pumps and the high bp and HR. but that just might be your natural test levels. so it would be pointless anyway. even a 5 digit increase in HR can totally cripple your times.