This is a tricky question to answer because there are many Camino routes that coalesce in Santiago. The 800 kilometer (500 mile) Frances route and the 825 kilometer (512 mile) Camino Norte route are probably the most difficult for sheer distance. Those Frances, Norte and Primitivo routes also have plenty of sections with steep hills. The Portuguese route is becoming increasingly popular. It covers 115 kilometers (75 miles), is more flat and works well for people with less time. But here's the thing. While the Camino is a physical challenge, it also presents mental challenges. Even if you aren't religious (and I'm not), the Camino gives you ample opportunity to shake the dust out of your psyche- but then you have to deal with what you find there. You have a lot of time to think while walking 20+ kilometers per day, which can be illuminating and creative, but also mentally taxing and likely to trigger some crying.