Walking is a great exercise. So, if you are up to it, you can do it. I did it once and I know it was difficult but you can do it if you are healthy and in great shape!
You can hire a tricycle up to Patad and walk all the way to Bulusan Lake. This will probably takes you at least two hours to walk and two hours walking back. So, you might like to start early. Add to that the 45 minutes walk around the lake.
Not sure how many kilometers or miles but I am pretty sure it took me at least two hours, non-stop, walking to the lake.
My tricycle driver told me that him and his tricycle fell down the ravine going to Bulusan Volcano Natural Park. He even pointed to me where he fell. Luckily, he said, he didn't have any injury nor a scratch!He was just lucky!
Some of the sides of the mountains are sliding and some of the roots of huge trees are blocking the shoulder of the roads. Be very careful especially when driving back because it gets too dark in there because of the huge trees!
Although it's a little bit expensive, make sure to have a contract with the tricycle driver and tell him to wait for you until you are done with your tour at the Bulusan Volcano Natural Park. Let's say, you wanted to kayak or to walk on the trail, tell him that you will be done in an hour or two and make this as part of your contract. Make sure not to pay the driver until you are done.
This way you are sure that he will not leave you by yourself in the park and you have a transportation back to the town!