According to Obama:
Someone who runs a marathon should be grateful since someone else ran those 26 miles.
Someone who lost a lot of weight via exercise should be grateful because someone else ran those miles, staved themselves, and exercsed.
Let's start with the fact that Romney agrees 100% with Obama's contention, as you can hear from Romney's own mouth in my previous post. Now, I will help your analogies actually make sense as per the speeches by Obama and Romney.
According to Obama (I assume you read and/or saw the full speech) and Romney, someone who runs a marathon "should be grateful" that they have roads and paths to run on that were paid for by taxes. If it is private property, then they should be grateful for the public roads they used to get to the marathon. If they were motivated by anyone to get into marathons, they should be grateful for those people. If they were motivated to run after seeing something on the internet, they should be grateful that the government created the internet.
According to Obama and Romney, someone who lost a lot of weight via exercise "should be grateful" because they learned how to do so via a book, t.v. show, the internet, family member, friend, tips on food products at grocery stores, etc. Moreover, the food they eat is regulated by the USDA.
In case you missed it (from July 18, 2012):