I've been down for a couple weeks with a chest thing and I can feel its taken its toll. I'm not sure if there's any way around losing fitness while being sick. When you immune system is down any exertion on it only makes things worse. Rest is the only way and resting is not riding, or even walking. I've been pounding my body with vitamins and healthy stuff and I still feel completely drained. The only thing you can do I believe is work your way back up the ladder. You could always try caffeine on your next ride to give you a little boost. There's a new thing out by Power Bar thats supposedly a good source of Vitamin C as well http://www.powerbar.com/products/19/POWERBAR®_GEL_BLAST_Lemon.aspx I dunno though overall I think it's just about building yourself back up after whatever has come in and messed ya up.