It's a bit of a trek...depending on where in the THV complex you wind up (and on how fast you walk), it could be as much as 20-25 minutes to Marketplace; 10-15 minutes to West Side...it's a nice walk, tho...
There are also buses that you can take from THV to either Grandstand or Springs, where you can get a Downtown Disney bus...
We're here at SSR now, in Grandstand...the Treehouse Villa buses come by quite frequently...
We have been walking to & from Downtown Disney frequently...at least once and sometimes twice each day..."for fun" the last 3 days we've timed our walks...it's about five minutes (walking at a normal, non-aerobic pace, but not "strolling") from Marketplace bus stop to the Congress Park pool...another 8 minutes to Carriage House, and another five minutes to our Grandstand building (first one across from the pool)...it was 20 minutes from the Pleasure Island bus stop back to our villa in Grandstand...THV would be another 5-10 minutes...
Hope this helps...