Just in case you were not able to use the chat - I am switching to Q&A mode and posting the answer below.
Please feel free to communicate if you need any clarification or have other tax
Does this "volunteer participant runner" receive a benefit per the IRS if he receives a T shirt to wear in the marathon?
Yes - the value of a T shirt is considered as receiving a benefit - and that value will reduce his/her deductible charitable contribution.
Are there any other benefits he is considered to receive?
According to the IRS - the donor can deduct only the amount that is more than the value of the privileges or other benefits the person receives. Thus - we need to determine the value of the privilege to participate in the marathon which includes the cost of a T shirt, the cost of food provided, etc. That is according to IRS rules.
As an illustration example...
Assuming registration fees to sign up for the Marathon are $100.
The organization cost of the Marathon - for instance - is $30 per each participant - we may use the cost to estimate the value of benefits received by the participant - that would be the value of the privilege to participate in the marathon.
So the remaining $70 will go to the Susan Komen Cancer Foundation for charitable purposes. Therefore - the deductible charitable contribution would be $70.