About 40 miles
I am guessing that it will be approximately 15mins-1h35mins. (roughly)
There isn't a direct one. As you are travelling between airports, I assume you've got baggage and will want to avoid long esculators and stairs. I recommend you take the London Underground Piccadilly...
Because there was a village (now under T3) called Heath-Row
25.45 kilometres (15.81 miles)