Ezra HaSofer enacted a takana than one may not learn Torah if tamei keri. The Gemora relates that the takana was too difficult to keep and was revoked. There are different minhagim regarding what people do today, I am writing the demands of Halacha and anyone whose minhag is more stringent should follow his minhag. There is a very important minhag to go to the mikva before Rosh HaShana and Yom Kippur and Mishna Berura writes very strongly that one should daven on these days in tahara. Otherwise there is no obligation in Halacha for a man to go to the mikva today.
There is an alternative when a mikva is unavailable of pouring 9 kavim of water in an uninterrupted flow over the body. This can be accomplished by taking a long shower.