Gradually return to your normal training regimen. You'll be slower of course, much slower at first, but you'll come around. I never ran marathons, more of a trackie, and didn't run another road race until 5 mo. post-AVR, then I stayed in the 3-3.5 mile range until a year out. I bumped it up to 10K, 8 mi & 15K over the next few months and returned to track races around 2 yrs out, for 800 m & 1 mile. Y'know where you get your HR right up there. It's almost 26 yrs now. I've since done 340 more races, but only one half-marathon, except in training where I'd do 3-4 of those a year, just because. I did a sprint Triathlon yesterday at age 69. Had I been 70 I would have won my AG, but too damn many younger 60s in it.
I ran occasional marathons, most well under three hours, and raced lesser distances more frequently, prior to my valve replacement. My increasing age and slowing race times had already led to a decrease in my motivation and attendance at races prior to my surgery but after my recovery I found that I had a renewed motivation to race, despite the fact that I was slower than ever. Since then I have run primarily 5K's and some 10K's with only one 10-miler and no marathons. This is because my weekly training is only around 25 miles a week with my longest training run around 8 miles. This is not sufficient to even think about running a marathon. I probably could if I wanted to up my mileage but I have also become so slow that I would be spending so much time on the road that I wouldn't have much of a life outside of my job. Maybe when I retire next year I will consider it. I'll also add that since my surgery I enjoy a built in excuse for being slow (although in reality my age has probably had the biggest impact) and I can enjoy racing without the added stress of trying to meet performance expectations. I do, however, still strive to place in my age group. Most importantly, I continue to run and go to races because of the strong sense of appreciation I have for the blessings of being able to run and to be physically active post open heart surgery. You should be able to return to the same type of training you were doing pre surgery within a year or so, just lower your expectations, and probably your pace.
I didn't do much serious running until after my AVR back in December, 2000. I started competing in sprint triathlons in 2004. Running was my weak discipline versus my swimming and cycling, so I started doing a lot more running during to the triathlon off-season. I eventually completed five half marathons and between 2009 and 2014 using Jeff Galloway's run/walk training plans. I used his beginner's plan the first year with the goal of just finishing, then moved up to his intermediate plan for the rest. I started suffering from some chronic knee and Achilles problems and decided to switch my emphasis to swimming two years ago.
As others noted, just by taking things easy, you'll be fine. I didn't do any real distance running until after surgery, and it worked fine to simply train thoughtfully without pushing things to a crazy degree. My cardiologist was fine with it, as I monitored my heart rate throughout. All the best with your half this summer.