I am working on endurance and strength. What dietary supplements can help?

I am running a Marathon next month and in 2 months I am doing a High-adventure Mountain Climbing race. Others doing the high-adventure mountain climbing race are taking Creatine and DHEA. Right now I only take Glucosamine and Chondroitin-Sulfate (for joints). Will Creatine and DHEA help with strength and endurance training. What other strength and endurance aids are out there?