Urgh. Not a good feeling. But if you think this could be the right one, you probably should try it again tomorrow and see how your body reacts. You don't want to build 40 days of practice around something, then have to kick it after 5 days of misery and then be all disappointed.

Alternately - you could choose this one, knowing you would fall back to a planned alternate and restart the 40 days if Invigorating made you too unhappy. Your psychological result could have a lot to do with how you approach it from the start.