i ran the 9km run this saturday with no prior training for a year, except riding bikes. i was pleased about the pace (4:15), however was hit with the most severe doms. i don't feel that i've got myself injured, but three days later the pain is really bad, i walk in small steps like an "old man" and walking down the stairs is quite a challenge.
Yep, that's what you get for running further than your body is used to. Cycling is great for cardio-vascular fitness and certain groups of leg muscles, but the quads and hamstrings are hit much harder by running than cycling.
If you'd been doing even 2-3 5km runs a week then the longer run is unlikely to have resulted in such severe (if any) DOMS.
Cold water on the legs (i.e. cool showers on the lower half or, even better, cool/cold baths) work wonders for me. I'd often come back from a long run and have a warm bath (keeping my legs elevated out of the bath) and then sitting upright, letting the water out slowly and turning the cold tap on and trying to stay in for at least 10 minutes.
At one point my old training regime would often have me doing a HM in the morning one day of the week and then going for an afternoon swim in the local pool whilst my daughter had her lesson. Even a 30 minute gentle swim in 26 deg C water was enough to bring my legs back to life.
If it's really bad then walking backwards down stairs works as it avoids reliance on those specific quads to take your weight each step.