What is going to kill you (or make the bike feel heavy) is rotating weight. Specifically, those HEAVY Marathon tires weigh somewhere near 740 grams each for 700x28. A lighter 'road' tire weighs somewhere near 280gr in the same 700x28 tire size (just so we're tanking an apples-to-apples comparison here). That 460gr per tire difference is A POUND per tire! Every tire rotation you are lifting that weight, then accelerating it to 2x riding speed (tire velocity with respect to the ground at the top of the tire). F=MA Force equals Mass times Acceleration. Simple physics means you are doing a LOT of work to accelerate that rotating weight. Of course the weight is killing you. If you were to get a nine-pound lighter bike and shod it with those heavy-tire 'combat boots', would you expect it to ride significantly different? The truth says otherwise.
Here's a little side-story that may serve as an example for you...
When I bought my '86 Miyata 710 off Craigslist while on vacation at my mother's place in Phoenix, it had a flat rear tire (I can't remember the brand) and a very skinny 23mm Vittoria Zaffiro on the front. The local bike shop only had a single Vittoria Randonneur 28 (similar to your bike's Schwalbe Marathon) in stock - and even that was over 2x internet price - yikes! I put that Randonneur on the rear along with a new tube and made the bike rideable at least - but it felt like a slug. AND it rode like a truck.
On my next visit to AZ, I packed along a pair of VIttoria Rubino Pro Tech tires and new matching tubes. Tires are ~280g each, and tubes are ~105g each, so less than 400 grams for tire and tube per wheel. When I went to remove the front tire, it had the the 400 gram wire-bead Zaffiro tire AND a heavy 'thorn proof' tube that was filled with Slime.That tube with slime weighed more than a pound by itself! The Randonneur tire that I had put on previously weighed ~700g plus the ~150gr standard tube. So total weight for both old tires/tubes was somewhere near 1.8Kg or close to FOUR POUNDS. New tires/tubes weighed 1.75 pounds total or 2-1/4 pounds less!!!
The difference in ride quality was remarkable! The lighter/more supple 150tpi (threads per inch) tires absorbed the road irregularities much better than the stiff slime-filled thorn tube/ 33tpi stiff carcass tires. Same frame, same wheelset, same other components. The only variable was the tires/tubes.
Last year, I was forced to get a new wheelset, and is even lighter by another half pound of rotating weight. Let me just tell you that I am VERY happy with my '80s steel bike now. Under 22-1/2 pounds now. Would I trade it for a newer 'wunderbike'? Not on your life!