You nailed it, this is where we are headed. Love to chat more about how you think we can get there.
You sometimes could have an open ended invite, and sometimes you would have to have both parties agree to meeting before hand, like in the case of restaurant reservations.
But running around the lake is not an open ended activity, because I don't want to wait around for people to show up.
Also add a button, "Leaving for the activity now", or "I'm here at X" would be helpful.