-(으)ㄹ 것이다 is the proper way to say it. Ending it with -(으)ㄹ 것, is not a full sentence and is not used in normal spoken or written contexts. - 나중에 달릴 것 (to run later) = a noun phrase, as might be used as a title, in personal notes, etc. - 나중에 달릴 것이에요/거예요 (I will run later) = full sentence. This is how we normally say or write it. As for 달리을 and 달릴, only the latter is correct. We never say 달리을. There are two cases to making a future form of a verb. 1. add 을 if the verb stem ends with a consonant. e.g. 먹다 (eat) -> 먹을, 짖다 (bark) -> 짖을. 2. add ㄹ if the verb stem ends with a vowel. e.g. 가다 (go) -> 갈, 따다 (pluck) -> 딸, 달리다 -> 달릴.

Actually there is no big difference between two sentences(will& be going to=both 저는 나중에 달릴 거예요) in korean. If you want to make difference, you can put some adverbs. And it is better to say ‘저는 나중에 달릴 거예요’ rather than ‘~것이예요’.