40km over 8 hours is just 5km/h
Looking back to when I first started cycling (and was unfit), even then I would average something between 15-20km/h. On that basis alone you should have ample time to complete your ride.
Texel is a beautiful place, there is little traffic (plus you will have many traffic-free, tarmac cycle routes) and it is perfectly flat. The only foreseeable problem will be the wind, so make sure you dress accordingly. To give an idea, I was staying once near Alkmaar, and rode up to Den Helder at around 40km/h. The return journey, however, was 20km/h - that is the difference the wind makes over there, and there really is no escape.
But having said all that, that sounds exactly like when an eBike would be useful.
Remember to carry some windproof gear.