If you have good walking shoes and aren't carrying a heavy load (cosplaying), the 1.3 mile walk should take about 25 minutes. The sidewalks in the area are pretty walkable. If you're weighed down a bit, it might be a little more work than you'd like, especially for a hot summer day. The weather is great at night though.
The Marriott and Hilton are so close you can probably travel between either hotel and the convention center within a minute if you moved with a purpose. You'll easily make it, unless you stop to take photos of cosplayers along the way or are stopped yourself. Trust us when we say it's close.