Dear Nizamuddin, I have travelled last year using the same route left Doha on mid day at 11:30 am and reached Riyad at 6 pm stayed there for a night and then left for Madina at 9 am next morning arrived at 5 pm in Madinah stayed for two nights and then traveled to Makkah. The road from Riyadh to madinah is very good and no curve or mountains' don't know but i think to cover it in a day will be exhausting with family.