Does it get easier?

I am waiting for it to get easier, but does it ever really? I know that the harder I have to work the more it is changing my body. As I was running today I was thinking back to last year when I could run a mile without stopping and even run a 5k without stopping. I was struggling this morning to run 5-6 min at a time before I felt like I couldn't go anymore. What gets me is that I know I have done it before so why can't I do it now? The running schedule I follow increases the running time a minute longer than the previous week. I find that I am not even getting used to running a certain time and then to add another minute seems so daunting-- how am I ever going to get back to running a full 5k without stopping? If I can't do that than how can I expect myself to complete a 1/2 marathon? On days like this I need to remember to stick with it, what I am doing will change my body, it will take time and hard work but it can be accomplished. I tend to give up too soon because I don't see the results, I need to trust in the process.