I had to take some time off this winter and have been trying to build my fitness back up this spring. For some reason, it just seems like my body is not adapting. I feel like shit pretty much every day, and I've increased my mileage pretty gradually. I've kept my easy days easy and I haven't even had any really tough hard days. I just feel crappy every day. I had my blood checked a while back and my iron and TSH were in the normal range... I'm only 19! Why can't my body get in shape as fast it used to?