I don't think this thread got the respect it deserved, so I'm commenting even if madarin will never see this. I came upon this quote while reading Running with the Buffaloes and decided to do some research which led me here.
From my understanding of the Latin language, word order is optional, which can often lead to confusion in translation. The context of the quote is usually the what determines the English translation. As is, either translation, "Only a serious thing is real joy" or "True joy is a serious thing" is appropriate. In its context, a letter by Seneca, the latter translation is more appropriate. Seneca is addressing Lucilius about joy and wants to emphasize that "True joy is a serious thing."
https://huehueteotl.wordpress.com/2007/03/23/res-severa-verum-gaudium/
By researching this, the quote for me began to carry much more meaning than just what Mark Wetmore gave it in "Running with the Buffaloes". In my opinion, the two translations play off each other: "True joy is a serious thing, but to find true joy, we must be serious."
I'm no Latin scholar and I am pushing my biased philosophy upon the translation, but hopefully my analysis will resonate with some running nerd out there.