25' of soakerhose might be enough for tempory watering system, if you have a very small area. I too have hard dirt and a small area. I use micro sprayers and run them for a couple of hours. They put out a heavy mist and therefore the water is able to saturate the ground and not run off. I got my system online at mrdrip. I have half circles running around the outside.

Your expert is sort of right. When water penetrates soil, it doesn't go straight down, it spreads wider in a teardrop shape. In most clay soils, water spreads 6-8" wide from the source on either side, a total of 12-16". So if you can place the soaker hose in a serpentine pattern with each leg being 12-16" from the others, it will work. Watch that you don't kink your hose when you are trying to make these tight turns. You might need to get some ground staples (or make some out of old wire coat hangers) to hold the hose in place.