Welcome Mr Green, very formal.
Just do it. Calculus is as hard as you make it. Meaning, that study and applying yourself fully is the way to get through it. It is not something to be feared. There is a learning curve that's kinda steep until it "clicks." From there on it is just hard work and effort applied.
I started out as a Toolmaker apprentice, then Draftsman and later a Design Draftsman. I got my Mech-Eng degree via night classes at a well accredited College and that took most of six years. Sometimes it was three nights a week and my body began to question if we had given up on sleep. I survived.
Without going back through the Calendar, I was in my late twenties, maybe early thirties when I started the night classes.
Anyway, instead of vacillating, get started now. The sooner the better as it is a great career. Starting at 35 means nothing detrimental, all the years it has taken you to get to 35, you must have learned a great deal along the way and all of that will only be a big help with the career path. Generally young Engineers fresh out of College and getting their first Engineering job are pretty useless for the first two years. You will have a huge leap start on them.
Good luck.