I think it's the chemicals in our diet that cause the tiredness. It must take a lot of effort for our bodies to remove them. I was virtually chemical free when I did my 30 day fast but had been eating junk food for several years when I began fasting again in August last year. On my 30 day fast I continued to work and be active all the way through it, but this time around I had to end my fast several times due to extreme symptoms and/or extreme exhaustion.
The choice is yours whether to end it or not. The tiredness will pass, there is just no telling when it will. If you end it at 7 days that is still an excellent rest for your body and great preparation for the next fast, especially if you refeed correctly with raw foods. The yoga was a great idea too, I hope you have the energy to keep fasting and do that too. But if you do end your fast don't think of it as a failure in any way, it's just a first step- and a pretty big one at that. :)