Why was the Crusaders' trip to the Holy Land so difficult?

A crusader leaving from Paris would have traveled more than 2,100 miles (3,360 km) to Jerusalem. That journey is roughly equivalent to walking from New York to Salt Lake City. This distance doesn't include numerous side journeys to forage for food or skirmish with enemy forces

Explanation:

Most pilgrims walked, and they had to climb steep mountains and cross semi-desert. The band tramped perhaps 12-15 miles per day, fewer in mountainous areas. Thus, the journey took many months. With sieges and delays for gathering food and supplies, most campaigns lasted for years. Thousands of crusaders deserted or died from disease, starvation, or warfare.