I'd say that would be a rough indication, but bear in mind that running on a treadmill tends to be slightly faster than running outside: firstly, the 'ground' under you is moving - when outside, you move and the ground says still. There are also things like inclines, wind resistance and running surface to take into account, all of which can make you slower. On a perfectly flat, tarmaced route I can run 5K in about 22 minutes, but my first 5K race, on a woodland, slightly undulating route with a few muddy patches, took me 26 minutes.
So don't be too disappointed or hard on yourself if you're not as fast outside as you are on a treadmill.