I get really exhausted and I find it difficult to move. My legs ache like crazy

Have a really good nap before you know its going to happen, or start walking and getting those legs in shape.

Tick

The funny thing is that my legs are fine, I can run, exercise and I don't feel exhausted. But when it come to bed I get exhausted easily

Could it be the position? Maybe it's time to try new positions. Or maybe stretch beforehand?

Maybe its time to find a new partner; sounds like boredom. Initiate some new life into your lovemaking. Take charge, more foreplay, make it interesting for your partner then it may be more interesting for you.l

You certainly don't have to have sex in a bed.
I fall asleep as soon as I hit a bed. I don't care how sexy someone is, I have to SLEEP in BED.
Take it from there.
If your legs hurt, you may have some problems with a sciatic or sacral nerve.
I had sacral nerve pain for years down my abdomen and inner thigh, to my big toe. A chiropractor helped a lot. After 30 years of MDs checking me for fibroids, hernias, ovarian cysts, and all that jazz.

Thank you :)