In reply to pancakeandchips:
Marks and Sparks hipster brief type things. Have lasted me for years. Tbf any underwear that's not too baggy seems to suit me, cotton or more man made. Running-specific pants strike me as more marketing innovation that development innovation, but have never tried them!
If the shorts come with integral supportive lycra/knicker lining I can go commando, but sometimes the material/seams can be a bit rough on the old danglies so I wear budgie smugglers for more protections
When I go longer and it's wet, or it's warm so I'll sweat (if it's really warm and I might take my top off - when out on the lonely bits of the moors I hasten to add - more sweat gathers in the shorts), I lube up with Vaseline on the inner thigh to reduce chaffing. Similar on other rub points like the armpits/bits of the back depending on pack I'm wearing