How is it that I lost so much fitness in such a short span of time???

There's a reason they call it peak fitness and not plateau fitness. That supreme endurance capacity you achieved by a big buildup, tapering and carb loading? It's gone. You lit a fire under it and burned it to the ground on race day, and then all the muscle and structural damage you took during the race left you in a hole. If you really got close to your marathon potential in your race, then recovery takes a lot longer than a week.

So yes, your slowdown is normal. It's not the time off that's hurting you. You may have actually needed more of it. I'd spend some more weeks on short, easy running before trying to get back into doing workouts.

Your heart rate is an indicator of fatigue, not just fitness. You are still fit, but you are recovering. Give yourself two more weeks.

Agree with DirtyDuck. The fitness is still "there." That's how people progress over time. However, peak training + marathon is asking a lot out of your body and you'll need to give it time to recover before you start seeing the same paces/HRs again. Some of it is also lost fitness, usually speed related, you can lose that sharpness pretty quickly. Aerobic fitness, not so much, I doubt you lost much at all. It will all come back quicker this time than it did before so long as you train consistently and get the rest/recovery you need.