I'm trying to be more out there since moving here, and I wanted to check out places like Krubb. I'm Young but not in great shape (more fat than muscle but not obese anymore) and I feel like I'm far from the Thai ideal of fitness. I don't know how welcomed I'll be somewhere like Krubb and I'm a bit nervous to check it out (I visited once last week but it was so early nobody else was there)

Try Kaikan Onsen first, there is a greater range of body types there. The vibe at Krubb depends on which night you go. 

It can be quite busy at Krubb Friday and Saturday evenings There will be men of all shapes, ethnicities and ages there, so don't worry too much.

I imagine that you are living closer to Krubb than to the Silom saunas. I used it a few times when I was staying in On Nut.