Hello!
No; you cannot go for 10km runs as diazepam has side effects of sleep. It can be dangerous and do not try this.
Drowsiness, dizziness and fatigue are common side effects of diazepam and will remain for a day after taking a dose of 5-10 mg.
Take care!
I'm no doctor, but I wouldn't recommend it. You won't be in full function of your body after taking a muscle relaxant and whilst running, I'd want to know exacty what was going on ...