The honest answer is: slower and shorter than you think, for longer than you want.
The return block we run twice a year is six weeks and starts at intervals of one minute running to two minutes walking. Experienced runners find the first fortnight insulting and the fourth week about right, which is the whole design.
You do not need to wait for a block to start. Come to any Sunday, walk the loop, and leave when you want to. Nobody will ask why and nobody will time you.