I think it is not easy for us(Japanese) to know the end of each words of hiragana, if there are long sentences using only hiragana. So we have to memorize kanji. It makes us easy to read Japanese sentences. For example, "あめのちくもり" is changed into "雨のち曇り", then you can easily understand this sentence has three words, "雨"＋"のち"＋"曇り".

If kanji isn't used and there is only hiragana without spaces, it really does become hard to tell what's a particle and where the start and end of a word is. When there is kanji, it becomes easier because there is a visual cue that it's not a particle anymore or that's the start of the new word. Since you have to work with hiragana, I suggest that you just take the reading slow to try and tell where each word begins and ends, and what everything in between is. For example, わたしのしゅみはほんをよむことです is easier to tell where each word is in this form: 私の趣味は本を読むことです.

I am still a big novice when it comes to learning Japanese but i try to look out for word the subject particles when reading kanji or hiragana. For example: は、の、え、に、が、と Are all particles that help me identify the end of a word. If you are reading text that is full hiragana try reading it out loud (or out loud in your head) and hopefully you should come across words you have heard before and be able to break them down when hearing them separately. I hope this helps, but my Japanese isn't very good so it may not be a approved approach!