Here's my solution to watching my husband's girls (10 and 12) during the summer when I have them every other week:
The mornings are understood to be a time to get some excercise. I'll either take them hiking or biking. They'll hike up to 6 miles and bike up to 18, and I vary the mileage. I prefer to hike with them because they are so slow on a bike and the hiking seems to keep me fit enough for the biking when I can get that in. I also vary the routine, sometimes I'll make oat bran/fruit muffins and pack juice and we'll eat breakfast as a picnic or while walking. Sometimes it'll be a lunch picnic. Sometimes I have a destination they enjoy, like a playground or the swimming pool (also good excercise) or grocery shopping (they like picking the groceries, checking off the list, etc) or the library. I began the summer by making a deal with them-no whining while doing what I like to do and we'll go do something free that they like in the afternoon. They got out of the habit of complaining and we all look forward to our mornings now (I don't let them watch tv or play on the computer during the day, I didn't like the absolute lack of family interaction or encouragement of couch potato-ness so we limit those activities, so there isn't so much of a draw to just stay home) at least more than before.
Things I've found that helps-
Something to drink!!! A camelback, a bottle, lots of waterfountains, drinking is a must, moreso than for me.
Snacks. Healthy stuff, of course. Their tanks run empty fast.
Expecting to not get to do as much as I'd like as fast as I'd like. Staying relaxed and focusing on having fun instead of how much we aren't doing.
Letting the kids have as much freedom as possible without putting them in danger and watching them to see what works best for them. The 12 year old especially is breaking away and trying to become her own person so I try to let her have more freedoms and responsibilities. The 10 year old rides faster if she's in front and lags really bad if she's in back. I keep stuff like this in mind.
Think about the things your daughter likes to do and incorporate those things into the excercise on occasion. Throw in a special treat destination every once in awhile, like a ride to get ice cream or something like that. Also think about her specific personality-what motivates her? Does she have a low pain tollerance like my youngest step-daughter? I have to be extra-carefull to keep her as comfortable as possible while we're out or she makes everyone's life miserable. Things that don't bother me in the least can be absolute agony for her and I have to keep reminding myself of that.
I don't know if you are able to or not, but I get up around 4:45 some mornings and hit the road from my house, riding until 7 am, when my DH has to go to work.
My 12 year old step-daughter is very competitive and likes to try racing me (part of the becoming her own person age), the other likes the idea of someone else doing the work and would really enjoy a tandem (though we can't afford one yet, working on it...).
Well, you get the idea. Sorry about the ramble! Best of luck in getting your daughter to excercise willingly, the key is in exploiting her personality, using her likes and dislikes to your (and her) advantage. I'm still working on it, but the girls have been doing a great job this year!
I used to have an open mind but my brains kept falling out.