In addition to what others have suggested, you may want to make sure you eat something with complex carbs before your walk. Ie: a little oatmeal or a small fruit smoothie (like no more than a cup). Your body needs that energy to get going and if you don't eat something before your workout, you may experience a lot of tiredness after. Just make sure you give yourself some time to digest it before heading out (you don't want stomach cramps on top of everything!)
I experienced the same thing this weekend when I did a seven-mile walk/jog. While I did have something before (a cup of Naked juice) it definitely wasn't enough because after my seven miles I just wanted to sleep the rest of the day. If I'd had a little more nutrition before, I'd have felt less tired.
If you're dog tired at night, though, that's a good thing. It'll help you sleep better, and I'm typically feeling that way in the evenings after a good workout earlier in the day.
Overall, though, exercise should leave you feeling alert and energized for at least a few hours after - not like you want to crawl back into bed immediately. I think the only exception to this is if you're pushing really hard and it's new to you. Your body's going to be a bit shocked at first.
Original weight in 2002: 220
Original weight when joining Spark 1/8/06: 149
| current weight: 131.0