I LOVE Radnor. Every time I go, I see deer, turkeys, turtles, owls, cranes and once I saw a SNAKE! I make it a point to go at least twice a month. One loop around the lake is just under 3 miles, but you can add on extra miles (about 6 total) by doing the extra trails.
One of the highlights of Radnor for me, is that every few months it's a completely different place.
In the summer the trees are full and it's very shady, autumn has scenery out of a movie. Winter is maybe not as gorgeous as the other seasons, but its neat to be able to see further into the forest part of the park and in the spring the fawns are born and all the flowers bloom (take your allergy meds!)
There is parking on either side of the park, but on a beautiful weekend parking fills up fast!