How much sunscreen?

I have been trying to go outside with minor success. I just don't want to spend money and somehow staying indoors is saving me money. 

Technically. 

You see I walk a dog or two and that is usually 30 minutes of pure undeterred sunshine. 

At work or my commute I walk about 40 minutes in the morning and 40 minutes early evening and 25 minutes from the bus stop to my house. 

Me just figuring out I walk 100 minutes a day about 5 miles and still haven't lost weight. 

So, I thought I could exercise a little in the backyard this past weekend. 

It was foggy in the morning and sunny in the afternoon. 

Those 15 minute chair exercises in the back yard may not be so helpful. 

If I lift a 5lb weight 20 times does that translate to lifting 100lbs? 

Anyway, I somehow someway got a sunburn not sure its because I didn't use any or the crook of my neck, elbow, knee or ankle just attract the sun more but I got a very weird stripping pattern and a lot of aloe vera gel.