There was a calculator I used to use to approximate time conversions at different weights based on changes in VO2max, but the link no longer works. Anyway, the physiological changes from lifting are likely to slow you down more than the added weight. It will take a while to get your muscles aerobically fit again. Trust me, I had to take off two months last year due to injury and my job required a lot of upper body lifting. I ran a 10k after two months of training and though I ran well given the amount of training I'd done, it felt like I was literally carrying 5lb dumbbells in each hand the last 2 miles. Beyond this, I typically gain weight in my chest and shoulders if I ever have to take time off from high mileage, and when I come back, my armswing just feels really awkward and sluggish.
Can't really say how much the 10lbs will slow you down. Upper body muscle is going to slow you down more than lower body muscle. As for losing it, 10lbs in one month is a LOT even if it was pure fat and you were 30lbs overweight. Two pounds per week is the max you should ever lose (after the first week), and even that would require 1000 Calorie defecit per day every day. Muscle takes longer to lose than fat, especially if you're not running a ton of miles. I would aim for two months, focusing more on gradually building up your miles than on actual weight loss. When I'm running a lot, I don't even have to try to lose weight. My chest shrinks naturally and I get a more streamlined physique. Just go out and run, avoid junk food, don't go crazy with the protein intake (1.8g or protein per kg of bodyweight is plenty). You'll get back if you want. Just be patient.