When I walk at 3,8 most of the time in the first 30 minutes I need to take breaks cause my legs hurts so much and I just can't go on. The muscles behind my tibia feels very hard. Is this normal ?

The same thing happens to me, wish I knew why!

I think you should listen to your body. You might have stressed a muscle, just to be safe I would check it out with a doc. I remember when my Achilles tendon was sore and after I rested for a week, it was much better, evidently I pushed myself too hard. And if you've been doing the same exercise for a while without complaint, it could your shoes are worn out.