Hi, good sentence but instead of "opgewonden" it is more common to use "veel zin". For example; ik heb veel zin om vandaag te rennen. In the second sentence you have to use the preposition "aan" and the article "het" before "regenen"; het is aan het regenen. Moreover, also use the article "het" before "rennen"; Het is aan het regenen maar dat is goed voor het rennen.

Ik heb erg veel zin om vandaag te rennen. Het regent, maar dan kan ik goed rennen. "opgewonden" can mean to be exited, but most of the time is it used in the sense of being turned on. Also opgewonden sounds a bit bookish, like someone who learns Dutch. Just say zin hebben when you would mean exited in English. "Het is regenen" we don't say that. The translation is too literal. Say het + present tense. "het regent". "dat is goed voor te rennen" it sounds like something I would say in my dialect and maybe someone who speaks a lot of dialect (Limburgish in this case) might say it like that, but I am 99% sure this sentence isn't grammatical in standard Dutch. Although I do understand what you mean. If you do insist on saying it like this don't forget "te". So "het regent, maar dat is goed voor te rennen" but it probably better if you just say "Het regent, maar dan kan ik goed rennen."