Sounds like she finds the walk boring.

Also are you walking on a gravel path or cement or something? her pads just might be extra sensitive with age, so those hard surfaces might be hurting her... try walking on the grass only & see how that goes.. cause you did say she likes to run in the dog park. I know when I walk on the sidewalk barefoot, it hurts my shins.. but if I walk on the grass, I'm fine.

One thing you could try, is get one of those tethers where you hook both dogs together... That'll give most of the work to Chipper & she just might work hard enough to keep with him, if it looks like hes the one leading her.