Don't worry about it. Sure, the run/bike combo will be harder overall than doing one alone with less volume, but you'll be in better condition because of it. Triathletes encounter this all the time, and it's normal. I run a marathon at near 7min/mile, yet in a olympic distance triathlon, after biking hard for an hour, it's hard for me to get to that pace in a measly 10k (6 mile) run. Just go with it.