You can order by phone or online in advance so you only have to walk to the restaurant to pick up your food.
Maybe you don't like the walking, but then think again: it has been shown many times that some physical movement during the working day is beneficial for your productivity and health.
If you really want to avoid leaving the building at all costs, there is also another option: ordering your lunch and let it be delivered.
Why are you assuming there even are restaurants near the office? A bunch of people getting into their cars and clogging up roads at lunch time doesn't sound like an improvement to me.
Yes, assuming a standard stride length of 0.7 metres and favourable traffic conditions, each lunch attendee could be expected to achieve 2900 steps. If travelling as a group there could be a minor social interaction every 4-10 steps thus leading to over 700 conversational actions which may help build camaraderie, strengthening team bonds and inproving the overall efficiency of the team.
If you really want to avoid leaving the building at all costs, there is also another option: ordering your lunch and let it be delivered.