Yes, mechs are able to walk backwards, but they are not able to run. Drakk, a big point on piloting a Heavy mech, especially a Marauder, its got a whole lot of armor, fighting in mechs as heavy as that is pretty much an endurance match, the job of the Heavies like that are to lead the charge, to get in close, relying only on your PPCs is not what the mech is for, it has medium and large lasers as well for when you get closer. You'll never be able to move fast enough to have a great chance at evading fire, smaller mechs can get in on you, and as fast as mediums and lights can go, its impossible to stay ranged, so you are better off engaging at long range with PPcs as you approach and then hitting with your closer weapons when they get in range since they are nowhere near as hot as the PPCs.
I think we may have given you the wrong idea about mechs Drak, heavy and Assault mechs can take insane amounts of punishment and carry large arsenals. While no mech can really laugh off all enemy fire, a light mech with medium lasers and machine guns, or even some medium mechs are no real threat, in the time it would take one to truly injure you, you would have blasted it into scrap.