Is Christianity based on older myths?

[Nope]

Jesus' teachings are very similar to a Jewish Rabbi named Hillel.

When a heathen who wished to become a Jew asked him for a summary of the Jewish religion in the most concise terms, Hillel said: "What is hateful to thee, do not unto thy fellow man: this is the whole Law; the rest is mere commentary" (Shab. 31a)

and

"Appear neither naked nor clothed, neither sitting nor standing, neither laughing nor weeping." Man should not appear different from others in his outward deportment; he should always regard himself as a part of the whole, thereby showing that love of man which Hillel taught. The feeling of love for one's neighbor shows itself also in his exhortation (Ab. ii. 4): "Judge not thy neighbor till thou art in his place" (comp. Matt. vii. 1). In the following maxim is expressed also his consciousness of his own insufficiency: "Trust not thyself till the day of thy death." How far his love of man went may be seen from an example which shows that benevolence must act with regard to the needs of him who is to be helped. Thus a man of good family who had become poor Hillel provided with a riding horse, in order that he might not be deprived of his customary physical exercise, and with a slave, in order that he might be served (Tosef., Peah, iv. 10; Ket. 67b).

Even if the word are not the same, the general lessons are very similar to what Jesus taught.

I have often wondered if the Jesus figure was just Hillel repackaged with mythologies that were popular at the time.

The bible even says that Paul was a student of Hillel's grandson, Gamaliel so there is a connection
http://jewishencyclopedia.com/articles/7698-hillel