I’ve rewrote my comment multiple times because I kept finding myself using too many words so here it goes as simple as I can get:

Lift. Sprint. Stretch. Absolutely destroy your legs and core in training. Build your back muscles - you can only accelerate your arm as fast as you can slow it down. You can jog if you want but sprints will benefit you more.

Long-tossing is great but throwing a baseball is one of the most unnatural motions that have become a normalized movement. You want to make it as easy as possible on your arm, so build up everything around it. Stretch your fingers and wrist. Stretch your neck. The more flexible your body is the less your pitching motion will tax your body.

I was a college baseball player and had tommy John - I trained my ass off in rehab and came back with 7mph added (88-95). Long-toss is great but if you’re overweight (I was that fat kid that was just good at baseball growing up haha) or just skin and bones, you aren’t going to be throwing a baseball as efficiently as possible.

TL;DR

Get in shape. Get strong. Get flexible. Velocity will be gained through this alone. Long-tossing is great as well. Video yourself pitching to find any inefficiencies and eliminate them.