Getting tired at trackdays

I've done about 9 trackdays so far, and im running into the issue of getting really tired to the point that i need to pit in after about 3-4 hot laps into the session. It's mostly leg pain, and just general exhaustion. I'm not that fast either, usually mid to upper b group pace.I work a desk job, so i don't get much exercise throughout the day, but i try to ride an exercise bike 2-3 times a week for 30-45 minutes each time. I've talked to people who barely exercise at all, but can do full sessions at a faster pace than me. Thinking about renewing my gym membership and committing 3-4 days a week to getting a full body workout. Has anyone had similar issues? And how did you get past them? via /r/Trackdays https://www.reddit.com/r/Trackdays/comments/b7dctk/getting_tired_at_trackdays/?utm_source=ifttt