Novice 40s / 6'1

Hi. I have recently dropped about 48lbs since 1st of September doing my own version of the Ketogenic diet, bike riding 3 - 5 times a week and starting to jog. I am now at a point that I am starting to hit the gym as I have gained willpower, motivation and starting to get, for me, fit.

I am following a gym program on different muscle machines that is fully body a 2 sets of 15 reps. After a couple of weeks on very low weights to learn to do them properly, I have started to reasonably push the weights up.

If I put on weights for each exercise that are challenging but sensible is it the same to do 2 sets of 15, or 6 sets of 5? I find with the 6x5 that I am able to challenge myself a bit more, keep myself actively under tension longer and do each rep much more slower.

It would be awesome to build a Chris Evans physique but basically I would like to gain some definition and put on some mass not primarily for looks but mostly to be fit, in shape and give me the tools from a power and endurance perspective to enjoy different sports (I currently mountain bike and the gym has greatly improved my climbing ability). Any advice? From a diet perspective I am no longer on Keto, I am just eating healthy but I am able to eat a lot and varied.

Thank you.