If you are not familiar with T5, you will need to get either the transit from T5A to T5C (well sign posted) or use the uncommon walking tunnels to reach T5C, if you fancy a fairly hearty walk, this is unusual in that only a few people use the tunnels and most (travelling public) do not know the tunnels exist! And HAL wouldn’t want hordes of people descending down there anyway.

Given you’re slightly nervous about the time constraints then follow the signs and crowds for the Transit to T5C. If you’re feeling fit and less anxious about the time, take a good walk to T5C before the long haul, I feel it’s helpful before mostly sitting for 6-10hrs! There are moving walkways to assist in the tunnels.