I always have walked alot. And I still do and I swim. I havent had any problems walking alot with my pregnancy. I have had swelling for a few weeks now mostly at night. I will ask my doctor about it at my 26 week appt in 2 weeks.
You should be ok, but like Petite said you should consult your doctor before doing any serious exercise during pregnancy just to make sure it's OK.
I know that I was walking about 2-4 miles a day in the weeks leading up to labor with my first child..mostly because I couldn't sit still, lol.
As long as you're drinking enough water and paying attention to warning signs (pain, cramps, shortness of breath) and stopping if your body lets you know it's having trouble, you should be OK until you can see your doctor and clear it - all strenuous activity and exercise should be cleared specifically by your doctor.
But hopefully he'll say it's ok and if he does, I say - walk on, lady! I am convinced that walking (and swimming!!!) during my first two pregnancies lead to easy and swift deliveries.
I would talk to your doctor about it because you are getting to the point where you may want to slow down a bit.. Swelling is normal but tends to be a symptoms that appears later on in the 3rd trimester.. though some people are just more prone to swelling..
I know that generally if you were walking 8 miles before you were pregnant, it should still be fine (you are just not suppose to introduce new activities that are strenuous)... As you get bigger over the next little bit and start experiencing more discomfort, you may find naturally you start to slow down .. But again at your next OB appointment ask about it tobe sure.