That's a long walk for your first time.
My personal experience with thousands and thousands of miles of walking and hiking is that on a regular surface (whether that's a gently graded trail, sidewalks or shopping malls) you have actually a better chance of feeling sore or getting weird pains here and there. The reason why is that all it takes is one little thing, perhaps a hot spot or blister on one heel or something like that, and you begin to favor that side. Your gait changes ever so slightly and with the even surface, every step you take is the exact same movement over and over again. The gait change gets beaten into you until you get a pain somewhere up the chain, in your shin, your knee, your hip.
So, always take good care of your feet. Wear shoes that are not too small. Try to walk on uneven surfaces to give your feet and legs little micro-breaks from doing the same repetitive motion over and over again. And just keep walking. Little minor aches aren't bad. It's the debilitating stuff you should be careful of. If it ever gets truly painful, take a break for a little while and go for a bike ride instead.
Female, 5'3", 48, Starting weight: 163lbs. Current weight: 135.
Starting squat: 45lbs. Current squat: 145 x 4.