why does working out make you feel old???

I wasn't sore at all on Monday after the half-marathon (was that only this past weekend??), so I cycled a bit. Tuesday I did speedwork, during which I felt pretty beat, but I finished fine. Wednesday I was all set to do 8 miles after work, and I didn't even make it through a mile before my shins started hurting. I'm starting to get a little nervous about injuries, so I jumped off the treadmill right away. I stretched alot, and yesterday I was a bit nervous to head out. My shin hurt for a little bit, but I ran on the (snow-covered) grass for a bit instead of the sidewalk, and it cleared up. My knee has been cracking this week, however. I had wanted to try to cut down on my glucosamine (I take triple strength daily), but as the cracking started at the exact time I started to cut down, I'm back to full-dosage today.

Boston is only 6 weeks away, tomorrow is our 18 miler. I'm a bit nervous about it, just b/c we didn't do a 16 miler, and the 15 was 3 weeks ago. I don't like all these creaks and cracks, I feel old! Tonight I'll have to dig out my log from the summer and see at what point I started to notice my toes cracking, etc. It sorta creeps me out, and I want to see if it was about this time in training last time. I know I'll run Boston, but I don't like to think about what I may be doing to my body in terms of long-term effects...